Interest in quantum dot mode-locked lasers (QD MLLs) has grown in recent years since their first demonstration in 2001
as applications for optical time domain multiplexing, arbitrary waveform generation, and optical clocking are
anticipated. Ultrafast pulses below 1 ps have been reported from QD MLLs using intensity autocorrelation techniques,
but so far detailed characterization examining the pulse shape, duration, chirp, and degree of coherence spiking in these
lasers has not been carried out. We describe the first direct frequency-resolved optical gating (FROG) measurements on
a QD MLL operating at a repetition rate of 5 GHz.
Higher-order harmonic repetition rate generation in quantum dot mode-locked lasers (QDMLLs) was realized using a
double interval technique. Using this approach, a wider operation range and improved mode-locking performance was
demonstrated for generating the 6th harmonic of the fundamental repetition rate. Without changing the layout of the
device, mode-locking at a repetition rate of 60 GHz, which corresponds to the 10th harmonic of the fundamental
frequency of the QDMLL, was achieved which cannot be realized utilizing the single absorber technique.
Monolithic InAs quantum dash 1.58-micron passively mode-locked lasers grown on an InP substrate are reported. A
repetition rate of up to 18.5 GHz has been realized. The dashes-in-a-well (DWELL) active region consists of 5 stacks of
InAs quantum dashes embedded in compressively strained Al0.20Ga0.16In0.64As quantum wells separated by 30-nm
undoped tensile-strained Al0.28Ga0.22In0.50As spacers on both sides of the DWELL. 4 micron-wide ridge waveguides with
cavity lengths in the range of 2.3 to 4 mm were fabricated with multiple electrically-isolated anode contacts. The modal
gain and loss spectra of the InAs active region were then measured through the improved segmented contact method, and
the characteristics that make InAs quantum dash materials system desirable for semiconductor mode-locked lasers were
identified. The segmented waveguides were then reconfigured into mode-locked lasers by wire bonding the segments
together to form separate gain and absorber regions utilizing the same DWELL active region. A highly reflective coating
(95%) was applied to the mirror facet next to the absorber while the other facet was cleaved. To assist in the cavity
design and to determine the relative length of the absorber and gain sections, a model for the cavity geometry of the twosection
passively mode-locked lasers was studied and is based on a microwave photonics perspective. A new set of
theoretical equations was used to find the optimal device layout using the measured modal gain and loss characteristics
as input data.
The dynamical response of a quantum dot photonic integrated circuit formed with a combination of passive
and active gain cells is investigated. When these cells are appropriately biased and positioned within the
multi-section laser cavity, fundamental frequency and harmonic mode-locking at repetition rates from 7.2
GHz to 115 GHz are found. Carefully engineered multi-section configurations that include a passive waveguide
section significantly lower the pulse width up to 34% as well as increase the peak pulsed power by
49% in comparison to conventional two-section configurations that are formed on the identical device
under the same average power. In addition an ultra broad operation range with pulse widths below ten
picoseconds is obtained with a 3rd-harmonic mode-locking configuration. The fundamental design
principles for using QDs in mode-locked lasers are presented to explain the observed results and to describe
why QDs are particularly well-suited for reconfigurable laser devices.
In this paper, two-section mode-locked lasers consisting of monolithic quantum dot gain and absorber
sections are studied as a function of absorber voltage, injected current to the gain region, and relative
section lengths. We map the regions of stable mode-locking as measured by the electrical and optical
spectra. A simple algorithm is presented that evaluates the quality of mode locking and allows automated
characterization of devices. The relative advantages of increasing the absorber length compared to
increasing the absorber reverse bias voltage are analyzed. Initial data indicate that doubling the absorber
length from 1.4 to 2.8-mm in a 5 GHz repetition rate device increases the region of stable mode-locking by
at least 25%, while increasing the absorber reverse bias can more than double the mode-locking regime.
Nonetheless, in these devices, stable mode-locking over greater than a 100 mA bias range is realized with a
grounded absorber making single bias control of a passively mode-locked semiconductor laser feasible.
In this work, the optical characteristics of monolithic passively mode-locked lasers (MLLs) fabricated from 1.24-&mgr;m
InAs dots-in-a-Well (DWELL), 1.25-&mgr;m InGaAs single quantum well (SQW), and 1.55-&mgr;m GaInNAsSb SQW
structures grown using elemental source molecular beam epitaxy (MBE) are reported. 5 GHz optical pulses with sub-picosecond
RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) were demonstrated in monolithic
two-section InAs DWELL passive MLLs. With the 42% indium InGaAs SQW MLL, a record high-temperature
performance for a monolithic passively mode-locked semiconductor laser is found. Compared with the typical operating
range of the InAs DWELL devices (<60°C), the operation is in excess of 100 °C. The first 1.55-&mgr;m GaInNAsSb SQW
MLL operates at a repetition rate of 5.8 GHz and has a 3-dB bandwidth of 170 kHz in the RF spectrum indicating
respectable jitter.
The modulation characteristics of multi-section gain lever quantum dot lasers are investigated in this paper. A 20-dB
enhancement in the amplitude modulation efficiency is observed in a two-section quantum dot laser. Based on rate
equation analysis a novel modulation response equation is derived to describe the device dynamics. In addition the
dependence of the modulation efficiency enhancement and 3-dB bandwidth on the length of the modulation section is
discussed. A conservative estimate of the gain lever value of 33 is derived from the measured results.
In this paper, we describe the results of using strain-compensation (SC) for closely-stacked InAs/GaAs quantum dot (QD) structures. The effects of the (In)GaP SC layers has been investigated using several methods. High-resolution x-ray diffractometry (XRD) quantifies the values of experimental strain reduction compared to calculations. Atomic force microscopy (AFM) indicates that the SC layer improves both QD uniformity and reduces defect density. Furthermore, increase in photoluminescence (PL) intensity has been observed from compensated structure. The use of Indium-flushing to dissolve large defect islands prevent further defect propagation in stacked QD active region. Room-temperature ground-state lasing at emission wavelengths of 1227-1249 nm have been realized with threshold current densities of 208-550 A/cm2 for 15-20 nm spacing structures.
For understanding the fundamental processes in QDs and optimizing the design of QD optical devices, it is essential to obtain accurate optical gain and absorption spectra. An improved segmented-contact method is described that subtracts the unguided spontaneous emission that normally introduces error into the calculated gain and absorption. Using the technique a QD gain spectrum is measured to an accuracy of less than 0.2/cm at nominal gain values below 2/cm. This capability also enables precise measurement of waveguide internal loss, unamplified spontaneous emission spectra and Stark shift data.
The correlations between the photoluminescence (PL) wavelength, integrated intensity, peak intensity, and FWHM with laser diode performance such as the maximum gain, injection efficiency, and transparency current density are studied in this work. The primary outcome is that the variation in PL intensity within a wafer originates primarily from differences in the radiative and non-radiative recombination rates and not from dot density variation. PL generated from 980 nm wavelength pumping appears to give more consistent data in assessing the optical quality of quantum dots that emit in the 1300 nm from the ground state.
Using the segmented contact method, we have measured the passive modal absorption, modal gain and spontaneous emission spectra of an InAs “dot-in-well” (DWELL) system where the inhomogeneous broadening is sufficiently small that the ground and excited state transitions can be spectrally resolved. The modal optical gain from the ground state saturates with current at a maximum value of one third of the magnitude of the measured absorption. The population inversion factor spectrum, obtained from the measured gain and emission spectra, shows that the carrier distributions cannot be described by a single global Fermi distribution. However, the inversion factor spectrum can be described by a system where the ground state and excited state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations.
Broad-area InGaAs/GaAs/AlGaAs double-quantum-well graded- index separate-confinement heterostructure diode lasers with non-intermixed and intermixed active regions were fabricated and characterized. Their light-current characteristics were used to extract information about the effects of impurity- free vacancy diffusion intermixing process on threshold current density, internal optical loss, internal quantum efficiency, material gain, etc. Comparison between these parameters indicates comparable device performance, even though lasers with intermixed active region underwent annealing at 1000 degrees C for 30 s and showed 42 nm wavelength blue shift.
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